dashcamd v3: trip directories, state machine, graceful shutdown

Dashcam recording now organized by trip in /data/media/0/videos/YYYYMMDD-HHMMSS/.
Starts recording immediately on launch (with 10-min idle timer), transitions to
continuous recording when drive gear detected. New trip on every ignition cycle.
Graceful shutdown via DashcamShutdown param with 15s ack timeout in thermald.

- Bitrate reduced to 2500 kbps (was 4 Mbps)
- Trip state machine: IDLE → RECORDING ↔ IDLE_TIMEOUT → TRIP_ENDED
- Deleter: trip-aware deletion (oldest trip first, then segments within active trip)
- camerad changed to always_run (was driverview) so dashcam works offroad
- DashcamShutdown param for graceful close before device power-off

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

common/params.cc

@@ -110,6 +110,7 @@ std::unordered_map<std::string, uint32_t> keys = {
     {"CurrentBootlog", PERSISTENT},
     {"CurrentRoute", CLEAR_ON_MANAGER_START | CLEAR_ON_ONROAD_TRANSITION},
     {"DashcamDebug", PERSISTENT},
+    {"DashcamShutdown", CLEAR_ON_MANAGER_START},
     {"DisableLogging", CLEAR_ON_MANAGER_START | CLEAR_ON_ONROAD_TRANSITION},
     {"DisablePowerDown", PERSISTENT},
     {"DisableUpdates", PERSISTENT},